We have studied the effect of a distribution function of ions disturbed by an electric field perpendicular to the geomagnetic field in the lower F region, on the ionic part of the spectrum of incoherently scattered radar waves. The distortion of the spectra from the shape they would normally have if the ion distribution were at local Maxwellian equilibrium, increases with the intensity of the electric field and with the angle of direction of observation of the radar with respect to the magnetic field. We determined as a function of the ion population and of the direction of observation the range of electric field values where the assumption of a bi-Maxwellian ion distribution is good enough to provide reliable measurements of the electron temperature and density and of the ion anisotropy temperature. Out of that range, the derived plasma parameters have large errors whose magnitude is related to the intensity of the electric field, to the ion composition, to the ratio of the electron temperature to the ion temperature, and to the direction of observation with respect to the magnetic field. ¿ American Geophysical Union 1989